these speakers pack a ton of low end, they are actually the big brother to the speakers you currently have in your amplifier. These are the speakers i use in my amp builds and i detune and play heavy all of the time.

Good choice of speaker. I was also going to recommend an E.Q. in the fx loop to boost low end. I had the 100 watt version of this amp. It too was thin sounding, I ended up selling it. One of the best modern Marshall combos I have heard has been the 50 watt JVM my buddy owns. It can be very thick and deep when needed.